Southern Pine is one of the hardest and most stable American softwood species and is perfect for framing and other structural uses. Southern pine is especially good for use in truss construction because its high density provides excellent plate-holding power for the metal plates that bind the trussesí chords and webs together. The Southern Pine lumber used for the floor trusses came from Temple-Inland Forest Products and the trusses were manufactured by American Truss Systems, Inc. For a portion of our roof, we used an engineered system of trusses also constructed using Southern Pine.
Parallel Chord Open-Web Trusses
All of the trusses to support the floors and roof of the Project House were engineered to fit the needs of this specific house. When a house is built using trusses, the builder sends the plans to the truss manufacturer, who then calculates the exact number and dimension of trusses needed to support the loads of that structure. The trusses are then fabricated to the specific dimensions needed, and numbered, so that when the trusses arrive on the jobsite, the framers will be able to put every truss exactly where it needs to be.
The floor trusses are constructed of a parallel top "chord" and bottom "chord" connected by a series of triangular structural "webs". The top chord is 3Ĺ inches across, which provides a wide nailing and gluing surface for supporting the subfloor.
The triangular webs shift the load laterally, which allows for construction designs that require supports that can span great distances. This feature is especially desirable in todayís residential construction, which tends to favor open rooms unobstructed by bearing walls, center beams and columns.
The open webs allow for mechanical features like electrical, plumbing and ductwork to be run through the structure. This eliminates the time, expense and danger of having to cut holes through the structural supports to make room for mechanical runs.
The trusses are relatively lightweight and easy to handle, and the ability to use continuous lengths means that there are fewer components to set.
Engineered Roof Trusses
Most homes built in the United States contain roof rafters that are put together on site by the framing crew. This method of construction has stood the test of time and has proven to be a reliable way to build a house.
However, there are various built-in limitations and uncertainties that accompany on-site rafter construction. How can a homeowner be sure that the work of the framing crew was consistent? Is the wood used for the rafters of a consistent high quality? Were the joints nailed together in a consistent manner? Were the boards cut so that the chords and webs fit flush at every joint? Are the rafters, as constructed, suitable for carrying loads that eventually may include not just the roof, but also several inches of snow? With an engineered roof truss system, all of these uncertainties disappear. Roof trusses are professionally engineered and manufactured to fit a specific need in a specific house. There is no guesswork in construction, and this includes specifications for high-quality lumber. In addition to this, because the trusses arrive on the jobsite pre-fabricated, the framers can assemble the truss system in a fraction of the time it would take to construct the rafters on site, by hand.
Perhaps the most interesting and useful feature of engineered roof trusses is the ability to create interior shapes that vary greatly from the standard triangular, "flat ceiling" profile. Would the architect like to create a vaulted ceiling? Because the trusses are engineered, they can be constructed in a manner that provides consistent support for the roof load, but allows for a curved "bottom". The interior space in the truss can be modified to create many interesting and useful shapes. For example, the attics of most homes built with rafters that are assembled on site are criss-crossed by timbers and provide limited freedom of motion in the attic space.
Engineered trusses can be built to create a wide open attic space, which allows for the creation of additional rooms and storage space that would not be possible using standard on-site rafter construction.